1. Field of the Invention
The invention relates in general to digital cellular communication systems, and more particularly, to soft handoff in spread spectrum communication systems.
2. Description of the Related Art
In a code division multiple access (CDMA) spread spectrum communication system, a shared frequency band is used for communication with all base stations within that system. An example of such a system is described in the TIA/EIA Standard TIA/EIA-95-B entitled "Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System", incorporated herein by reference. The generation and receipt of CDMA signals is disclosed in U.S. Pat. No. 4,401,307 entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITES OR TERRESTRIAL REPEATERS" and in U.S. Pat. No. 5,103,459 entitled "SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", both of which are assigned to the assignee of the present invention and incorporated herein by reference.
Radio Frequency (RF) signals are exchanged between a respective mobile unit and one or more base stations. Mobile units do not communicate directly with one another. Base stations communicate with a base station cellular or personal communication system controller, referred to herein as a base station controller (BSC) using various media such as ground based wires or a microwave link, for example. The BSC can route calls to a public switching telephone network (PSTN) or can route packets to a packet switched network, such as the Internet. The base station also coordinates the operation of base stations within the system during soft handoff for example.
TIA/EIA-95 is one example of a CDMA communication system. Communication from a mobile unit to one or more base stations in a TIA/EIA-95 CDMA system takes place over shared frequency channels each of which occupies approximately 1.25 MHz of radio frequency bandwidth. More specifically, communication signals occupying a given frequency band are discriminated at a receiving station through the spread spectrum CDMA waveform properties based on the use of a high rate pseudonoise (PN) code. A PN code is used to modulate signals transmitted from the base stations and mobile units. Signals from different base stations can be separately received at a given mobile unit by discrimination of different PN codes. For the TIA/EIA-95 standard, these codes are constructed from a single code, but each base station has a unique time offset of the PN code. The high rate PN spreading also allows a receiving station to receive a signal from a single transmission station where the signal has traveled over distinct propagation paths. Demodulation of multiple signals is disclosed in U.S. Pat. No. 5,490,165 entitled "DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS" and in U.S. Pat. No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both of which are assigned to the assignee of the present invention and incorporated herein by reference.
The various channels within a given "forward" (base station to mobile unit) TIA/EIA-95 CDMA channel include data channels, a synchronization channel, a pilot channel, and a set of paging channels, all transmitted from the base station to mobile units. The pilot channel carries a pilot signal that is a regularly repeated pattern that is differently phase offset for each base station. The pilot provides for time reference and for amplitude and phase tracking. The pilot signal allows mobile units to identify and become synchronized with the various base stations that are within range of their communication capability. The synchronization channel carries additional synchronization information for use by mobile units. The set of data channels carry the data associated with the various communication sessions (usually phone calls) and are directed to individual mobile units. These data channels are called traffic channels in TIA/EIA-95. The paging channels are used by the base stations to notify mobile units when a request to communicate has been received.
The protocol for paging a subscriber unit typically is defined so as to evenly distribute power transmitted by the base station over time, and to reduce power consumption in the mobile unit. In some CDMA systems, the mobile unit power consumption while monitoring the paging channel is reduced by dividing each base station's paging channel into a set of paging "slots" or time windows during which paging signals directed to mobile units may be generated, and by assigning a particular CDMA frequency channel, paging channel and paging slot to each mobile unit in an evenly distributed fashion. This assignment generally may be performed, for example, via the use of a hashing function applied to the International Mobile Station Identity (IMSI) assigned to each mobile unit, although other unique numbers could be utilized. The hashing function of set of hashing functions yields a value that corresponds to a particular set of paging parameters including a CDMA channel, paging channel, and paging slot. Mobile units are then configured to "listen" for a paging message over their assigned CDMA frequency channel and paging channel during the corresponding paging slot. Since listening for a page requires a certain amount of power, limiting the time a mobile unit performs the listening function to a particular paging slot reduces the overall power consumption of that mobile unit and therefore increases the life of any battery or other power storage system utilized by that mobile unit.
During a mode of operation called "soft handoff", the mobile station simultaneously exchanges identical communications traffic on a CDMA traffic channel between two or more different base stations. In the case of (forward link) reception by a mobile station, the signals from the multiple base stations can be coherently combined to improve performance, as with multipath combining. In fact, the second base station signal can be regarded as a delayed version of the first, generated actively and purposely, rather than as a delayed reflection of the first caused by the environment. See, CDMA, Principles of Spread Spectrum Communication, by Andrew J. Viterbi, Addison-Wesley Pub. Co., 1995, pp. 181, 183-184, 198-199 and 222-224. Soft handoff is further disclosed in U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM" and U.S. Pat. No. 5,267,261 entitled MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM", both assigned to the assignee of the present invention and incorporated herein by reference. Similarly, a mobile unit can simultaneously communicate with two sectors of the same base station, known as softer handoff, as disclosed in copending U.S. Pat. No. 5,625,876 entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION", assigned to the assignee of the present invention and incorporated herein by reference. Handoffs are described as soft and softer because they make the new connection before breaking the existing one.
In a typical CDMA communication system, mobile unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the mobile unit. In the TIA/EIA-95 CDMA communication system, for example, an Active Set is the set of base stations through which active communication is established. A Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication. A Candidate Set is a set of base stations having a pilot signal strength of sufficient level to establish communication.
In at least one earlier CDMA system, for example, soft handoff can be set up between a mobile unit and two or more base stations in the mobile unit's Active Set. For instance, in such an earlier CDMA system, when communications are initially established, a mobile unit typically communicates through a first base station, and the Active Set contains only the first base station. The mobile unit monitors the pilot signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. When a pilot signal of a base station in the Neighbor Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the mobile unit. The mobile unit communicates a message to the first base station identifying the new base station. A base station controller decides whether to establish communication between the new base station and the mobile unit. If the base station controller decides to establish communication, then it sends a message to the new base station with identifying information about the mobile unit and a command to establish communications therewith. A message is also transmitted to the mobile unit through the first base station. The message identifies a new Active Set that includes the first and the new base stations. The mobile unit searches for the new base station transmitted information signal, and communication is established with the new base station in soft handoff (without termination of communication through the first base station). This process can continue with additional base stations.
When the mobile unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold for a predetermined period of time, the mobile unit generates and transmits a message to report the event. The base station controller receives this message through at least one of the base stations with which the mobile unit is communicating. The cellular or personal communication system controller may decide to terminate communications through the base station having a weak pilot signal strength.
While soft handoff generally has been successful on dedicated channels such as traffic channels between a mobile unit and one or more base stations in an Active Set, soft handoff generally has not been available on common channels such as the paging channel, for example. One reason for the absence of soft handoff on a typical paging channel is that soft handoff ordinarily requires sending identical messages from every base station participating in the soft handoff. In prior systems, however, this could result in a considerable reduction in paging channel capacity. For example, let the number of base stations involved in soft handoff on the paging channel be N. Then the paging channel capacity could be reduced by a factor of N. In even a small city, there are tens of cells; and the paging channel capacity could be substantially reduced. Since all base stations in a network cannot be in soft handoff, then there would probably have to be boundaries between groups of cells, and the performance between these groups of cells would be reduced. It is possible to use sets of overlapping groups of cells which are in soft handoff. However, this could mean that many messages would have to be transmitted by multiple sets of these cells, thus resulting in lower paging channel capacity. Unfortunately, since the paging channel generally is not operated in soft handoff, fading and shadowing can cause the forward link of one base station to become stronger than the forward link of another base station.
This can result in significant problems when the mobile unit is attempting to perform system access since it generally cannot as readily perform (hard) handoff in the System Access State. Moreover, calls often are dropped due to delays in getting the mobile unit into soft handoff after the traffic channel is set up.
A similar problem exists when the mobile unit is first assigned to a traffic channel, and the mobile unit initially receives the traffic channel from only one base station. This similar problem was addressed by a technique included in the TIA/EIA-95-B standard, called "Soft-Channel Assignment." Soft Channel Assignment is disclosed in U.S. Pat. No. 6,021,122, issued Feb. 1, 2000, entitled METHOD AND APPARATUS FOR PERFORMING IDLE HANDOFF IN A MULTIPLE ACCESS COMMUNICATION SYSTEM, assigned to the assignee of the present invention and incorporated herein by reference. Soft Channel Assignment reduces the problem of delays in getting a station into soft handoff following traffic channel assignment. During such Soft Channel Assignment, a Channel Assignment Message carries a list of base stations which should be in the mobile station's Active Set. By doing Soft Channel Assignment, the mobile unit is placed into soft handoff as soon as it is assigned to the traffic channel. This significantly increases the speed in which the mobile unit can be placed into soft handoff, thus increasing the reliability of the call set up.
Various techniques have been proposed in the TIA/EIA-95-B specification to reduce the incidence of dropped calls resulting from fading of the paging channel. Two such techniques are called, "Access Probe Handoff" and "Access Handoff". The basic principles are disclosed in U.S. Pat. No. 6,021,122, issued Feb. 1, 2000.
Access Probe Handoff permits a mobile unit to switch to monitoring a new base station between access probes. The mobile unit would switch to monitoring a new base station whenever the forward link of the base station that the mobile unit is monitoring becomes too weak. Thus, for instance, if the mobile unit does not receive an acknowledgement to an access probe, and the mobile unit determines that the paging channel is weak, then the mobile unit may shift to using a new base station. An Extended System Parameters Message sent by the base station to the mobile unit informs the mobile unit of whether it is permitted to perform an Access Probe Handoff and the set of base stations to which the mobile unit is permitted to perform the Access Probe Handoff. In one earlier embodiment, for every base station in the mobile station's neighbor list, the Extended System Parameters Message has a one-bit flag that indicates whether an Access Probe Handoff is permitted to that base station.
Access Handoff permits a mobile unit to switch to monitoring a new base station while awaiting a Channel Assignment Message. Thus, for example, if the mobile unit has received an acknowledgement to its access probe, and the mobile unit determines that the paging channel is weak, then the mobile unit may shift to using a new base station. An Extended System Parameter Message sent by the base station to the mobile unit informs the mobile unit of whether it is permitted to perform an Access Handoff and the set of base stations to which it is permitted to perform the Access Handoff In one earlier embodiment, for every base station in the mobile station's neighbor list, the Extended System Parameters Message has a one-bit flag that indicates whether an Access Handoff is permitted to that base station.
Yet another earlier method specified in TIA/EIA-95-B, referred to as, Access Entry Handoff, permits a mobile unit to begin monitoring a new base station from the time in which the mobile unit receives a page until it transmits a Page Response Message.
Traffic channel handoff generally requires the mobile station to receive an explicit message instructing the mobile unit to perform the handoff. In Access Probe Handoff, Access Handoff, and Access Entry Handoff, the mobile unit does not receive an explicit message instructing it to handoff, but performs a handoff autonomously to a limited set of base stations about which it has informed the base station and about which have been pre-enabled by the base station.
In order to facilitate Soft Channel Assignment, Access Probe Handoff, and Access Handoff, in accordance with the TIA/EIA-95-B specification, the Access Channel Messages sent by a mobile unit identify the set of base stations whose received signal strengths are above a certain threshold level, called T_ADD in TIA/EIA-95-B. The base stations having strengths above T_ADD are those base stations that would be typically placed into the Active Set if the mobile station were on the traffic channel. In TIA/EIA-95-B, the mobile station reports two different lists of base stations in the Access Channel Message. One list is the IDLE_HANDOFF_LIST. This is the set of base stations for which the received signal strengths exceed T_ADD and for which the mobile unit is allowed to perform an Access Probe or Access Handoff. The latter is controlled by one-bit flags in an Extended System Parameters Message corresponding to neighboring base stations. A second set of base stations are those for which the signal strengths exceed T_ADD, and for which Access Probe or Access Handoff is not allowed. The base station can use this information when performing soft channel assignment.
The illustrative drawing of FIG. 1 provides a hypothetical example of the operation of Access Probe Handoff, Access Handoff and Access Entry Handoff.
Nevertheless, there still have been problems with lost calls due to the absence of soft handoff on common channels. For example, there have been problems with lost calls due to loss of the paging channel during call setup. This results in annoyance to customers of the system. Furthermore, the amount of power required to be transmitted on the paging channel is quite large, thus reducing the overall capacity of the system.
Thus, there has been a need for soft handoff on common channels, such as the paging channel, in a cellular system, such as a spread spectrum communication system. The present invention meets this need.